Measurable Snow with Rain Category
66% of the total events produced measurable snow during the 6 hour period
o 65% of the measurable snow events occurred with rain
o 35% of the measurable snow events were all snow
17% of the total events were rain mixed with trace snow.
17% of the total events were freezing rain
17 total events
Principal forecast issues
Near freezing isothermal segments in polar air masses
· Observations of sounding characteristics within 24 hours of the onset of precipitation leading to measurable snow from rain and snow mixtures has shown that some polar air masses are characterized by near freezing isothermal segments, typically somewhere in the 900-800 mb layer.
· The presence of these segment(s) may well be an air mass precursor to the formation of a near freezing isothermal layer supporting measurable snow from mixed rain/snow events.
· The measurable snow with rain predominant 6 hour interval p-type category is defined on the predominant p-type nomogram by the partial thickness values of 1290/1537 – 1298/1532 – 1312/1530 – 1312/1538 – 1290/1542.